U.S. patent number 4,413,181 [Application Number 06/287,134] was granted by the patent office on 1983-11-01 for arrangement for stroboscopic potential measurements with an electron beam testing device.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Hans-Peter Feuerbaum.
United States Patent |
4,413,181 |
Feuerbaum |
November 1, 1983 |
Arrangement for stroboscopic potential measurements with an
electron beam testing device
Abstract
An electron beam testing device system for stroboscopic
potential measurements of a test subject utilizes a scanning
electron microscope having a beam suppression or blanking system.
The blanking system deflects the electron beam across an aperture
during each edge of a blanking pulse connected to control the
blanking system so that two electron pulses are generated for each
blanking pulse. A detector produces a signal responsive to a
secondary electron beam resulting from impact of each of the
electron pulses on the test subject. A signal processing unit with
an associated gate circuit all preferably incorporated in a boxcar
integrator processes only one of the two electron pulses associated
with each blanking pulse. A phase control preferably within the
boxcar integrator is connected to control the gate circuit and a
blanking pulse generator for producing the blanking pulse.
Inventors: |
Feuerbaum; Hans-Peter (Munich,
DE) |
Assignee: |
Siemens Aktiengesellschaft
(Berlin & Munich, DE)
|
Family
ID: |
6113107 |
Appl.
No.: |
06/287,134 |
Filed: |
July 27, 1981 |
Foreign Application Priority Data
|
|
|
|
|
Sep 29, 1980 [DE] |
|
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3036660 |
|
Current U.S.
Class: |
250/310;
324/754.22; 324/762.02 |
Current CPC
Class: |
H01J
37/045 (20130101) |
Current International
Class: |
H01J
37/04 (20060101); G01N 023/00 () |
Field of
Search: |
;250/310,311,306,307
;324/158D |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
J Phys. E: Sci. Instrum., vol. 11, 1978, Article entitled "A
Sampling Scanning Electron Microscope", at pp. 229-233, Gopinathan
et al., and also article entitled Beam Chopper for Subnanosecond
Pulses in Scanning Electron Microscopy, at pp. 529-532, Feuerbaum
et al..
|
Primary Examiner: Anderson; Bruce C.
Attorney, Agent or Firm: Hill, Van Santen, Steadman &
Simpson
Claims
I claim as my invention:
1. An electron beam testing device system for stroboscopic
potential measurements of a test subject, comprising: a scanning
electron microscope having a beam suppression or blanking system in
which an electron beam is deflected across an aperture in the
blanking system during each edge of a blanking pulse connected to
control the blanking system such that two electron pulses are
generated per blanking pulse; blanking pulse generator means for
producing said blanking pulse; detector means for producing a
signal responsive to a secondary electron beam resulting from
impact of each of the electron pulses on the test subject on which
the potential measurements are being made; a signal processing
means with an associated gate circuit for processing only the
signal associated with one of said two electron pulses associated
with each blanking pulse; and phase control means connected to
control said gate circuit and said blanking pulse generator
means.
2. A system according to claim 1 wherein the signal processing
means with its associated gate circuit comprises a portion of a
boxcar integrator.
3. A system according to claim 1 wherein a switching means is
associated with said pulse generator means for selectively
providing said blanking pulse with different reference potentials
such that said blanking system produces either said two electron
pulses for each side edge of the blanking pulse or one of electron
pulse corresponding to a width of the blanking pulse.
4. A system according to claim 1 wherein the blanking system
includes a first deflection plate which is connected to a reference
potential and a second deflection plate connected to receive said
blanking pulse.
Description
BACKGROUND OF THE INVENTION
The invention relates to an arrangement for stroboscopic potential
measurements with an electron beam measuring or testing device
which exhibits a beam suppression blanking system, and in which the
electron beam is deflected across an aperture or diaphragm during
each edge of a sampling pulse such that two electron pulses are
generated for each sampling pulse.
High frequency signal paths can be stroboscopically measured with
an electron beam testing device. Accordingly, a finely focused
electron beam which is directed onto the measuring subject, for
example an integrated circuit, serves as the test probe. Due to the
interaction between electrons and solid bodies, secondary electrons
among other things are released which can be employed for imaging
an object. These secondary electrons also carry information
concerning the electrical potential at the location of incidence.
Upon exploitation of the stroboscopic effect, specimens or subjects
being tested functioning with a high nominal frequency can also be
quasi-statically imaged as a potential contrast. For this purpose,
the measuring subject to be examined is targeted with cyclically
repeating signals and is imaged in a scanning electron microscope.
The electron beam is turned on only once for a brief time in each
cycle, i.e. the measuring subject is only observed during a
specific phase. Thus, the imaging is a snapshot of the rapidly
functioning probe. The point in time at which the electron beam is
switched on can be selected at random within the cycle. Slowmotion
presentation of the switching operations is possible by means of
slowly shifting the phase. The on-time duration of the electron
beam can be reduced down to the picosecond range, i.e. the
chronological resolution of said imaging method lies in the
picosecond range. The electron range are generated with the
assistance of a beam suppression or blanking system.
A purpose of the device disclosed herein is to generate the
electron pulses and to process the secondary electron pulse
signals.
In order to generate short electron pulses, deflection structures
(plate capacitors, traveling wave arrangements) which deflect the
electron beam from the beam path onto a diaphragm or aperture were
previously employed. There are fundamentally two possibilities for
the drive of the deflection structures. According to method I (the
arrangement specified by H. P. Feuerbaum and J. Otto in J. Phys. E:
Sci. Instrum., Vol. 11, 1978, 529-532, incorporated herein by
reference, and which can be employed for this purpose), one
deflection structure is grounded, whereas a constant voltage is
applied to the other deflection structure. A blanking pulse
superimposed on the constant voltage places the deflection
structure at grounded potential. During the blanking pulse, the
space between the two deflection structures is field-free, so that
the electron beam is switched on. In this method, the electron
pulse width is determined by the width of the blanking pulse
applied. The smallest electron pulse width hitherto attainable
amounts to 350 ps.
According to method II (K. G. Gopinathan, A. Gopinath, "A Sampling
Scanning Electron Microscope", J. Phys. E: Sci. Instrum., Vol. II,
1978, 229-233, incorporated herein by reference, shorter pulse
widths (.apprxeq.10 ps) are achieved when one deflection structure
is grounded, whereas a negative constant voltage -U.sub.o is
applied to the other. A blanking pulse superimposed on the constant
voltage places this deflection structure approximately at the
potential +U.sub.o. Therefore, the electron beam is deflected
across a diaphragm or aperture during each edge of a blanking
pulse. In accordance with the rising edge and falling edge, the
circuitry supplies two electron pulses per blanking pulse.
Since a stroboscopic measurement, however, requires one electron
pulse per signal cycle with which the test subject is driven, this
wiring is only suitable when
(a) one of the two electron pulses is deflected out of the beam
path by means of an additional beam blanking system; that, however,
is only possible at great mechanical and electronic expense.
(b) two respectively successive electron pulses have the same
chronological spacing with respect to one another and the frequency
of the signal with which the test subject is cyclically driven is
twice as high as the frequency of the blanking pulse. In this
method, however, a high chronological resolution can be achieved
only after a protracted adjustment of the spacing of the two
electron pulses.
SUMMARY OF THE INVENTION
An object of the invention is to provide an arrangement for
stroboscopic potential measurements with an electron beam testing
device which renders possible a high chronological resolution
without additional expense with respect to the signal processing or
with respect to the beam suppression or blanking system.
This object is inventively achieved in that the testing device of
the invention employs a signal processing device with a gate
circuit which, for each blanking pulse, only measures the signal
caused by one electron pulse. Thus, in the signal processing, a
chronological window is placed in such manner that only the signal
of one electron pulse is measured per blanking pulse. A boxcar
integrator is advantageously employed for such a signal processing
device.
The inventive arrangement can advantageously be switched between
the aforementioned operating modes according to a first method I
(FIG. 2, hereafter) and according to a second method II (FIG. 3
hereafter) in a simple manner.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an inventive arrangement for stroboscopic potential
measurements with an electron beam measuring or testing device;
FIG. 2 shows the circuitry of a beam suppression or blanking system
according to method I;
FIG. 3 shows the circuitry of a beam suppression system according
to method II; and
FIG. 4 shows the principle of the stroboscopic potential
measurement in an arrangement according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, an inventive arrangement for stroboscopic potential
measurements with an electron beam measuring device is illustrated.
An electron beam measuring device according to the cited prior art
which is to be inventively modified can be employed as the basis
for an inventive arrangement. The basic unit forms a modified
scanning electron microscope 16 with a fast beam suppression system
1. This beam suppression or blanking system 1 exhibits the two
deflection structures 2, 3. In order to be able to stroboscopically
measure the high-frequency periodic events in the test object or
subject 5, an electron beam 4 of primary electrons is pulsed
synchronously with the event, so that it strikes the test object 5
only during a specific phase of the high-frequency periodic event.
The potential in this phase of the high-frequency periodic event is
determined with the assistance of a spectrometer from the energy of
the secondary electrons 6 which are released in pulse-shaped form.
For this purpose, the signal of the secondary electrons 6 which
have passed through the spectrometer are amplified in a
scintillator/photomultiplier combination 7.
Accordingly, a modified boxcar integrator 14 assumes the control of
the phase and the signal processing. A boxcar integrator is an
amplifier which samples high-frequency signals with a time window
(gate) according to the sampling principle. The boxcar integrator
14 exhibits a phase control unit 8, a signal processing unit 9, and
a gate circuit 10. After amplification in the
scintillator/photomultiplier combination 7, the signal of the
secondary electrons 6 arrives at the input of the gate circuit 10.
The values measured at a specific phase during the high-frequency
periodic operation in the test subject 5 are integrated in the
signal processing unit 9 and are subsequently amplified. For
employment in electron beam testing or measuring technology, the
following change must be carried out at the boxcar integrator 14: a
delay circuit 11 (approximately 150 ns) is built in between the
phase control unit 8 and the gate circuit 10. Without delay, the
phase control unit 8 drives the pulse generator 12 of the beam
suppression system 1. The signal of the emitted secondary electrons
6, as a result of the transit time of the primary electrons 4,
appears with a delay at the input of the gate circuit 10. At the
same time, when the signal of the emitted secondary electrons 6
appears at the input of the gate circuit 10, the gate circuit is
driven via the built-in delay circuit 11 in such manner that the
signal of the secondary electrons 6 is amplified. The boxcar
integrator supplies a signal 15 which is proportional to the signal
level of the secondary electrons 6. The overall arrangement also
exhibits an electronic auxiliary control device 13 for the drive of
the test subject 5 and for the drive of the phase control unit 8 in
correspondence thereto. Such a device is well known to those
skilled in this art.
FIG. 2 shows the circuitry and applied voltages of a beam
suppression system 1 according to the aforementioned method I. The
deflection structure 2 is grounded, whereas a constant voltage is
applied to the deflection structure 3. A blanking pulse
superimposed on said constant voltage places said deflection
structure 3 at ground potential. During the blanking pulse, the
space between the deflection structures 2, 3 is field-free, so that
the primary electron beam 4 is switched on. The electron pulse
width in this method is determined by the width of the blanking
pulse applied. The smallest pulse width previously attained amounts
to 350 ps.
FIG. 3 shows the circuitry and applied voltages of a beam
suppression system 1 according to the aforementioned method II as
employed in this invention. The deflection structure 2 is again
grounded, whereas a negative constant voltage -U.sub.o is applied
to the deflection structure 3. A blanking pulse superimposed on
said constant voltage places said deflection structure 3 at the
potential +U.sub.o. During each edge of a blanking pulse, the
electron beam 4 is deflected across a diaphragm or aperture 17. In
accordance with the rising edge and falling edge of each individual
blanking pulse, this system supplies two primary electron pulses 4
per blanking pulse. With this system, shorter pulse widths can be
achieved (.apprxeq.10 ps).
FIG. 4 shows the principle of the stroboscopic potential
measurement with an arrangement according to the invention. The
deflection structures 2, 3 are wired according to the method II
(FIG. 3) so that two electron pulses 4 are generated per period of
the high-frequency periodic event in the test subject 5. In the
signal processing unit 9, a chronological window Z is established
in such manner that only the signal of a single secondary electron
pulse is measured per period of the high-frequency periodic event
in the test subject 5.
FIG. 4 shows the voltage U applied to the deflection structure, the
pulsed primary electron current I.sub.PE generated therewith, the
pulsed beam current I.sub.PE of the primary electrons 4 generated
by means of the voltage U at the beam suppression system 1, and the
pulsed beam current I.sub.SE of the secondary electrons 6 which
results. This signal (current I.sub.SE) arrives at the gate circuit
10 with a delay as a result of the transit time of the primary
electrons 4.
In comparison to the known arrangements, the inventive arrangement
has the advantage that one can switch, in a simple manner, between
the operating modes according to method I and according to method
II by simply providing a switching circuit in the pulse generator
for applying the potential shown in FIG. 2 rather than FIG. 3. The
device Model 162 of the PAR company was employed as the boxcar
integrator having the above described gate circuit, signal
processing unit, and phse control unit therein.
In the previously cited article "Electron-Beam Testing of VLSI
Circuits", IEEE of Solid-State Circuits, 471-481, in FIG. 5 of this
reference, there are described the peripheral accessories required
for the stroboscopic voltage contrast mode and for the sampling
mode according to this invention. In reference 20 of this cited
article, (H. P. Feuerbaum et al, "Beam Chopper For Subnano-Second
Pulses In Scanning Electron Microscopy", J. Phys. E: Sci Instr.,
Vol. 11, 1978, 529-532), an appropriate beam chopper system (pulse
generator 12 and suppression system 1) as well as an appropriate
electronic device 13 are described.
In a preferred embodiment, as the electron microscope 16, a
modified ETEC autoscan scanning electron microscope may be employed
with a large-area specimen chamber in which a printed-circuit board
has been placed on the x,y stage (P. Fazekas et al, "On-Wafer
Defect Classification of LSI-Circuits Using A Modified SEM",
SEM/1978/801-806), the test subject 5 under test is inserted in a
socket on this board, which is implemented as a personality card
for each test subject (such as an integrated circuit) to be
inspected. Coaxial cables run from the printed-circuit board
through coaxial vacuum feedthroughs to a second printed-circuit
board, which is likewise provided with line drivers. Connected to
the second circuit board is a HP Microprocessor-lab 5036A as the
control device 13 which drives the test subject 5 under test.
Located above the microprocessor under test is the electron
spectrometer for voltage measurements (H. P. Feuerbaum "VLSI
Testing Using The Electron Probe", SEM/1979/I, SEM INC. AMF O-Hare,
IL6066 (1979), 285-296).
The inventive arrangement can also be additionally operated with
symmetrically wired deflection structures 2,3, as is described by
H. P. Feuerbaum and J. Otto in the above cited publication. During
blocking operation, the one deflection structure then lies at the
potential +U.sub.o, whereas the other deflection structure lies at
the potential -U.sub.o. The primary electron beam 4 is switched
since the deflection structures 2, 3 have applied thereto a zero
potential by the blanking pulse.
Although various minor modifications may be suggested by those
versed in the art, it should be understood that I wish to embody
within the scope of the patent warranted hereon, all such
embodiments as reasonably and properly come within the scope of my
contribution to the art.
* * * * *